1. Field of the Invention
This invention relates generally to turret moored Floating Production Units (FPUs) and in particular to lower bearing arrangements which provide radial sliding support by which a vessel can weathervane, that is rotate, about the turret.
2. Description of the Prior Art
U.S. Pat. No. 4,955,310 provides a lower radial bearing arrangement between a turret and an opening of a lower support arm which is secured by the vessel. The lower bearing arrangement permits the lower part of the turret to pivot about a pair of horizontal axes. An upper bearing arrangement is also provided such that the upper bearing can pivot about a pair of horizontal axes. The turret is free to move axially (at least to a small extent) through the lower bearing arrangement at the interface between the turret and the bearing pads. The outer housing for the lower bearing is rigidly attached to the vessel.
U.S. Pat. No. 5,515,804 discloses a resiliently mounted upper bearing arrangement which is an improvement of the ""310 patent mentioned above. Lower bearing segments are illustrated which allow the lower bearing to tilt about a horizontal axis to allow it to follow the turret when the turret wobbles during turning due to misalignment.
U.S. Pat. No. 5,316,509 discloses a lower bearing assembly. Bushing segments are carried by a bushing block which is rollingly supported from a support block fixed to a structural support of the lower turret. The bushing must be radially adjusted when the turret is inserted within the lower bearing assembly. In other words, the turret is inserted into the bearing upon installation.
U.S. Pat. No. 5,782,197 discloses a lower bearing arrangement between a moonpool of a vessel and the lower portion of a turret. A plurality of segment structures are provided where each segment includes a base and an elastic body that supports the bearing segment while allowing it to move radially or tilt.
A disadvantage of the bearing arrangements of the prior art patents ""310, ""804 and ""509 identified above is that the lower bearing outer housings are rigidly secured to the vessel""s structure (i.e., moonpool) with elastomer material provided between the outer housing and the bearing surface. Because the turret is arranged to move axially within the lower bearing arrangement and to react against frictional loads, such designs are arranged to limit shear forces on the elastomer materials.
All of the lower bearing designs described above call for the bearing assembly to be secured to the bottom of the moonpool prior to the installation of the turret into the moonpool and within the outer housing of the lower bearing assembly. Adjustment of the bearing segments must then be made at a place which is difficult to reach by workmen and at a time in the installation process which is costly.
3. Identification of Objects of the Invention
A primary object of this invention is to provide a new lower bearing assembly for a turret moored Floating Production Unit (FPU) vessel that is mounted to the turret instead of to the vessel""s hull structure while allowing the outer housing of the bearing to xe2x80x9cridexe2x80x9d or move with the turret while the turret undergoes wobble (that is, small tilting) motion due to structural deflections and other influences at the top of the turret.
Another object of the invention is to provide a coupling between an outer housing of the lower radial bearing of the turret that is coupled to a reaction ring of the moonpool so that rotational force of the vessel is transferred to the outer housing such that radial bearing support is provided between the turret and the moonpool.
Another object of the invention is to provide a coupling between an outer housing of the lower radial bearing of the turret which includes a curved profile of an outer surface which contacts an inner surface of a reaction ring on the moonpool such that contact occurs between the outer and inner surfaces when the turret wobbles or tilts with respect to the vessel.
Another object of the invention is to provide an outer housing of a bearing structure which is arranged and designed to fit about an exterior ring around the turret and to allow for the installation and capturing of segmented bushings between the turret ring and the outer housing such that the bearing assembly can be mounted on the turret before installation of the turret in the moonpool of the vessel.
Another object of the invention is to provide an outer housing of a lower bearing assembly that is secured to the turret and has an outer rim with an outer facing curved profile arranged and designed to slide and/or roll with respect to a cylindrical surface of a reaction ring in the moonpool of the vessel thereby emulating a spherical joint.
The objects identified above along with other features and advantages are provided with a lower bearing arrangement that is fixed axially to the turret shaft. The lower part of the turret may xe2x80x9cwobblexe2x80x9d or pivot about the center line of the moonpool. Because the lower bearing assembly is fixed to the bottom of the turret, it xe2x80x9cridesxe2x80x9d on a reaction ring installed in the moonpool when there is turret wobble.
The lower bearing assembly of the invention includes an outer housing which is mounted on an inner ring of a cylindrical section which can be welded in the lower portion of the turret. The inner ring alternatively can be secured about the end of the lower turret. A segmented bushing is secured about the inner periphery of the outer housing such that the bushing of the outer housing has rotating sliding contact with the inner ring surface of the turret. The outer housing is mounted with respect to the turret such that it is free to rotate about the lower turret, but it is not free to move axially with respect to the turret.
The outer housing has an outer rim with a curved profile. A reaction ring, having a surface which faces the curved surface of the rim, is rigidly attached to the inside of the cavity of the moonpool. The rim of the outer housing is free to slide and/or roll a small amount on the reaction ring when the turret wobbles or pivots with respect to the vertical axis of the moonpool. The reaction ring is keyed to the outer housing so that the rotation of the moonpool (when the vessel weathervanes) is transferred to the outer housing.